DETRITAL-ZIRCON RECORD OF PROVENANCE OF ORDOVICIAN–DEVONIAN SANDSTONES IN THE APPALACHIAN FORELAND

The Ordovician sedimentary record of Taconic accretion on the Alabama promontory of the Iapetan margin of Laurentia includes early proximal black shale with sandstone interbeds (Athens Shale) and later distal redbeds and sandstone (Colvin Mountain Sandstone). The two sandstones have similar detrital-zircon populations: dominantly ~925–1300 Ma, less abundant ~1300–1400 Ma, and rare older ages to 2700 Ma. Two Colvin Mountain zircons have ages of 585 and 671 Ma.

Northeast along the orogen, detrital-zircon ages reported for Ordovician (Fincastle, Oswego, Austin Glen) and Silurian–Lower Devonian (Shawangunk, Tuscarora, Rose Hill, Keefer, Oriskany) sandstones are similar to those of the Ordovician sandstones in Alabama, except for more abundant ages of ~1330–1560 Ma. A few zircons have ages of 460–480 and 665–690 Ma in the Oswego, 720–730 Ma in the Austin Glen, and 553 Ma in the Tuscarora.

To the west along the Ouachita orogen, detrital-zircon ages reported for Ordovician (Blakely) and Silurian (Blaylock) sandstones are similar to the new results from Alabama, except that Blaylock has one zircon at 486±51 Ma and no zircons older than 1400 Ma. Granite boulders in the Blakely have ages of 1284–1407 Ma.

Zircon ages from the Devonian Frog Mountain Sandstone in Alabama are similar to those of the Ordovician sandstones, except for more abundant ages of 1250–1500 Ma. Two Frog Mountain zircons have ages of 582 and 697 Ma. In strong contrast, Devonian sandstone in the Gondwanan Suwannee terrane has zircon ages almost exclusively of ~530–825 and ~1875–2170 Ma.

Zircons older than ~925 Ma in the Appalachian Ordovician–Devonian sandstones imply recycling from Iapetan synrift and passive-margin rocks, which contain detritus from Laurentian cratonic provinces, as well as primary sources in Appalachian Mesoproterozoic massifs. The few zircons with ages of 550–730 Ma may have sources in either Iapetan synrift igneous rocks or accreted Gondwanan terranes, which are indistinguishable by age. Only the 460–480 Ma zircons in the Oswego suggest a Taconic synorogenic provenance. The 486±51 Ma zircon in the Blaylock may have a Taconic or Iapetan synrift/Gondwanan source. The Taconic foreland of Laurentia received detritus from orogenic uplifts, but detritus from synorogenic crystalline rocks or accreted Gondwanan terranes appears very rare.